This invention is concerned with devices, such as adjustable mirrors, smart windows and optical attenuators, for controlling the reflectance and/or transmission of electromagnetic radiation.
Sunlight transmitted through windows in buildings and transportation vehicles can generate heat (via the greenhouse effect) that creates an uncomfortable environment and increases air conditioning requirements and costs. Current approaches to providing xe2x80x9csmart windowsxe2x80x9d with adjustable transmission for use in various sunlight conditions involve the use of light absorbing materials. Such approaches are only partially effective since the window itself is heated so that heat is transferred into the interior by convection. In addition, these devices, such as electrochromic devices, are relatively expensive and exhibit limited durability and cycle life. Certain liquid crystal-based window systems switch between transmissive and opaque/scattering states, but these systems require substantial voltages to maintain the transparent state. There is an important need for an inexpensive, durable, low-voltage smart window with variable reflectivity. Reflecting the light, rather than absorbing it, is the most efficient means for avoiding inside heating. Devices for effectively controlling transmission of light are also needed for a variety of other applications. For example, an effective means for controlling light transmission over a wide dynamic range is needed to permit use of inexpensive are lamps as light sources for projection displays.
Bright light from headlamps on following vehicles reflected in automobile rear and side view mirrors is annoying to drivers and creates a safety hazard by impairing driver vision. Currently available automatically dimming mirrors rely on electrochromic reactions to produce electrolyte species that absorb light that would otherwise be reflected from a static mirror. Such devices do not provide close control over the amount of reflected light, and are expensive to fabricate since a very constant inter-electrode spacing (i.e., cell gap) is required to provide uniform dimming. Image sharpness is also reduced for electrochromic mirror devices since the reflected light must pass through the electrolyte (twice). There is an important need for an inexpensive adjustable mirror device that provides close control of reflected light with minimal image distortion.
There have been attempts in the prior art to exploit reversible electrodeposition of a metal for light modulation, primarily for display applications [see for example, J. Mantell and S. Zaromb, J. Electrochem. Soc. 109, 992 (1962) and J. P. Ziegler and B. M. Howard., Solar Eng. Mater. Solar Cells 39, 317, (1995)]. In this work, metal, typically silver or bismuth, was reversibly electrodeposited onto a transparent working electrode, usually indium tin oxide (ITO), from a thin layer of electrolyte sandwiched between the working electrode and a counter electrode. Both water and organic liquids (e.g., dimethylsulfoxide or dimethylformamide) were employed as solvents. The deposits obtained on the transparent electrode presented a rough and black, gray, or sometimes colored appearance (typical of finely-divided metals) and were used to enhance light absorption by display elements. Pigments were often added to the electrolyte to provide a white background for improved contrast. An auxiliary counter electrode reaction (e.g., halide ion oxidation) was typically employed to provide a voltage threshold (which is needed for matrix addressing) and/or to avoid metal deposition on a transmissive counter electrode (which would offset the light modulation provided by metal deposition on the working electrode). Such auxiliary reactions introduced chemistry-related instabilities during long term operation and led to deposit self erasure on open circuit via chemical dissolution of the metal deposit. Nonetheless, the key drawback of reversible metal electrodeposition for display applications was the relatively slow response for attaining adequate light blocking.
A reversible electrochemical mirror (REM) device permitting efficient and precise control over the reflection/transmission of visible light and other electromagnetic radiation is described in U.S. Pat. Nos. 5,903,382, 5,923,456, 6,111,685 and 6,166,847 to Tench et al. In this device, an electrolyte containing ions of an electrodepositable metal is sandwiched between a mirror electrode and a counter electrode, at least one of which is substantially transparent to the radiation. A typical transparent mirror electrode is indium tin oxide (ITO) or fluorine doped tin oxide (FTO) deposited on a transparent glass (or plastic) pane which serves as the substrate. Application of a voltage causes the electrodepositable metal, e.g., silver, to be deposited as a mirror on the mirror electrode while an equal amount of the same metal is dissolved from the counter electrode. When the voltage polarity is switched, the overall process is reversed so that the electrodeposited mirror metal is at least partially dissolved from the mirror electrode. A thin surface modification layer of noble metal, e.g., 15-30 xc3x85 of platinum, on the transparent conductor is usually required to improve nucleation so that a mirror deposit is obtained. The thickness of the mirror metal layer present on the mirror electrode determines the reflectance of the device for radiation, which can be varied over a wide range.
The REM technology can be used to provide control of either light reflectance or transmission, or both. A transmissive REM device suitable for smart window applications utilizes a noble metal counter electrode that is locally distributed, as a grid for example, on a transparent substrate, e.g., glass or plastic, so that mirror metal deposited thereon does not appreciably increase the blockage of light. In this case, high light transmission is provided by a locally distributed counter electrode of relatively small cross-sectional area and the device reflectance/transmission is adjusted via the thickness of mirror metal on the mirror electrode. As described in U.S. Pat. No. 6,166,847 to Tench et al., such a transmissive counter electrode is not required for reflective REM devices used for adjustable mirror applications. An electrolytic solution, which provides the inherent stability, high deposit quality, complete deposit erasure, long cycle life, and reasonably fast switching needed for most practical applications, is described in U.S. Pat. No. 6,400,491, to Tench et al. This solution is typically comprised of 1.5 M AgI and 2.0 M LiBr in a gamma-butyrolactone (GBL) solvent, and may also contain highly dispersed silica (HDS) added to produce a gelled electrolyte and/or dispersed carbon added to blacken the electrolyte so as to reduce background light reflection.
A significant problem with both electrochromic and REM devices is that light modulation at constant applied voltage occurs more slowly toward the center of the device. This reduced modulation rate results because the voltage is decreased by the relatively high sheet resistance of the transparent conductor film, e.g., indium tin oxide, which is used for at least one of the electrodes. Such xe2x80x9cirisingxe2x80x9d is most noticeable for low light modulation states and is unacceptable for many applications. The iris effect can be mitigated by utilizing lower switching currents, for which the Ohm""s law (IR) voltage drop is less, but at the sacrifice of switching speed. Switching speed of electrochemical light modulation devices is also limited by the need to avoid excessive voltages at the electrode interfaces with the electrolyte, which can cause decomposition of the electrolyte or damage to the electrode surfaces. A means for uniformly switching REM devices at relatively fast rates would greatly increase their utility and provide an additional advantage compared to electrochromic devices.
The method of the present invention provides uniform switching at relatively fast rates for reversible electrochemical mirror (REM) devices, which are comprised of an electrolyte containing electrodepositable metal ions, e.g., silver ions, in contact with a mirror electrode and a counter electrode. The electrolyte may be a liquid or solid electrolytic solution, an ionic liquid electrolyte, or a solid electrolyte. A stiffening agent may be added to render a liquid electrolyte more viscous, semi-solid or solid. The mirror electrode is typically comprised of a very thin layer of noble metal (e.g., platinum) on a layer of a transparent conducting oxide (e.g., indium tin oxide) on a glass or plastic substrate. Generally, the counter electrode is a sheet or layer of the electrodepositable mirror metal for devices that are designed to control radiation reflection, and is a locally distributed electrode for devices that also transmit radiation. The device reflectance is determined by the thickness of the mirror metal layer on the mirror electrode, which can be adjusted by applying a voltage of the appropriate polarity to cause mirror metal electrodeposition or dissolution, while the reverse process occurs at the counter electrode. The present invention exploits the fact that the sheet resistance of the mirror electrode decreases as the thickness of the deposited mirror metal layer increases. This sheet resistance decrease is unique to REM devices and provides another significant advantage compared to normal electrochromic systems.
According to the method of the present invention, improved mirror uniformity with minimal sacrifice in switching speed is attained for REM devices by utilizing lower drive voltages when the sheet resistance of the mirror electrode is high, and increasing the drive voltage when the sheet resistance is reduced by an appreciable thickness of the mirror metal. Good mirror uniformity is provided since the resistive voltage drop along the mirror electrode surface is minimized by the lower currents when little or no mirror metal is present, and by the low sheet resistance when the mirror metal thickness is appreciable. The overall switching time can be short since the current, which is directly related to the switching rate, can be greatly increased for thick mirror metal deposits without inducing mirror nonuniformity. The improvement provided is greatest for reflective-type devices with continuous metal counter electrodes also having low sheet resistance. However, the invention is also useful for transmissive-type devices utilizing counter electrodes that are locally distributed or located outside the light path.
Large voltages that would otherwise decompose the electrolyte or damage the electrode surface can be applied to increase the switching rate when current is flowing. This is because the voltage drop associated with the resistance of the electrolyte does not appear as electrode potential across the electrode-electrolyte interface. Consequently, the drive voltage can be increased beyond a safe value for the electrode potential by the magnitude of the resistive voltage loss (IR drop) in the electrolyte without detrimental effect. Likewise, the drive voltage is decreased so as to limit the electrode potential to a safe value as the current decreases in the later stages of mirror erasure. Such IR-compensated device switching is another aspect of the present invention.
In a preferred approach, the REM device is automatically switched (via a computer) according to a drive voltage algorithm based on real-time measurements of the electrode sheet resistance, device switching current and temperature. A method for measuring the electrode sheet resistance, which also yields the device reflectance/transmission, is described in U.S. Pat. No. 6,301,039 to Tench. Typically, the computer memory contains data defining the device current as a function of voltage and temperature, as well as the mirror electrode sheet resistance as a function of temperature. This data can be in the form of equations (and appropriate constant parameters) since the voltage drop in the electrolyte is typically much larger than the potential drops at the electrodes so that the device current varies linearly with the applied voltage (to a good approximation). Since the reciprocal of the electrolyte resistance is typically linear with temperature, a simple equation can also be used to determine appropriate adjustments in the applied voltage to compensate for changes in the device temperature. As a key feature of the present invention, the device current, preferably for both plating and erasure, is limited so that the voltage drop along the electrode (current x sheet resistance) does not exceed a value chosen to provide the best compromise between mirror uniformity and switching speed.
A variety of alternative approaches within the scope of the present invention will be apparent to those skilled in the art. For example, the charge passed in electrodepositing mirror metal on the bare electrode provides a measure of the deposit thickness that could be used to provide feedback on the mirror electrode sheet resistance in real time. In principle, the electrode sheet resistance could be known at any given time via the thickness of the mirror metal deposit by utilizing a charge integration device and keeping track of all of the charge passed for metal electrodeposition and dissolution as the mirror state was cycled. However, as the mirror was subjected to multiple cycles in which complete erasure of the mirror metal did not occur, measurement imprecision and minor efficiency imbalances between the metal electrodeposition and dissolution reactions would introduce cumulative errors in the calculated thickness and associated reflectance. However, this approach could be used with devices for which the mirror deposit is fully erased on a frequent basis.
In another embodiment of the present invention, a drive voltage that varies with time is used and no sheet resistance feedback is needed. In this case, a relatively small negative voltage is applied to initiate mirror formation and the voltage is stepped or ramped to more negative values as the mirror metal is deposited and the electrode sheet resistance decreases. Likewise, a relatively large positive voltage is applied to initiate mirror erasure and the voltage is stepped or ramped to less positive values as the mirror metal deposit is dissolved and the mirror electrode sheet resistance increases. Excess applied voltage to compensate for the electrolyte IR drop could also be used in this case. This approach is most appropriate with devices for which the mirror is fully erased on each cycle, as is typically the case for smart windows.